For high performance hard disk drive assemblies, very fast rising and falling times and precise impedance control are required in order to maximize the performance of write drivers. Low power supply voltage is generally preferred to avoid potential problems with excessive power consumption and heat dissipation. Conventional series-type impedance control circuits have problems including, but not limited to, power supply voltage, or “headroom” degradation due to a voltage drop over matching resistors placed in series. In addition, write current tolerance is limited by the inevitable variation in the impedance of internal series resistors.
Another challenge faced by series-connected impedance control circuits known in the arts is their response to changes in the write current. The write current naturally tends to be dependent upon the write head impedance. Referring to the representative prior art circuit shown in FIG. 1, write current IW that flows into the write head is determined by the ratio of the internal reference resistance Ro and the external write-head-impedance (Rh+Rfpc). Because of this, write-current is dependent on not only the inherent variations present in the internal resistance, but also on the write-head-impedance. Increasing the internal reference resistor Ro decreases the write current's dependency on the write-head-impedance, but detrimentally increases the voltage Vh drop across the write head.
Swing voltage asymmetry is another problem faced by write driver circuits known in the arts. Asymmetric swings in head voltage can result in loss of stored data and damage to the heads. Typically between a preamp and a recording head (i.e. write head), and a reading head (not shown) there are wires or lines for both writing and reading. Due to area constraints, the lines are typically closely spaced, e.g. on the order of about 50–100 um, making them susceptible to “write-to-read coupling” or “crosstalk” resulting from mutual interference. The swing voltage of the write lines can be very fast and large in magnitude, sometimes causing damage to the heads through capacitive and/or inductive coupling between the write lines and read lines.
These and other problems encountered by prior art conventional write driver circuitry, such as that represented by the example shown in FIG. 1, make it difficult to provide both current-accuracy and high-performance while also using a low voltage power supply. It would be useful and desirable in the arts to provide improved impedance-controlled write driver apparatus and methods. It would be advantageous to provide a reliable and accurate impedance-controlled write driver circuit for use with a low voltage power supply. Advantages would also be obtained by avoiding the reduction of headroom and making head voltage swings symmetrical.